A boiling water reactor (BWR) includes a pressure vessel containing a nuclear reactor core above which is disposed a conventional steam separator assembly and in turn a steam dryer. The vessel is partially filled with water to a normal level above the core and within the steam separator assembly, with the core being effective for heating the water therein to generate steam which rises upwardly into the steam separator which removes a portion of moisture therefrom, with the steam dryer removing additional moisture prior to discharging the steam from the pressure vessel for powering a steam turbine for example.
In order to maintain the normal level of water in the pressure vessel, the level must be monitored so that various systems may be actuated when required as the water level varies. In order to measure the water level, the fundamental fluid hydrostatic relationship between pressure differential relative to height in a liquid reservoir is used. A reference leg or pipe is joined to the pressure vessel above the water normal level and contains a substantially constant column height of water therein, and a variable leg or pipe is also joined to the vessel below the normal level with a conventional differential pressure monitor or level transmitter being joined between the two legs. By measuring differential pressure between the two legs, the level of the water in the vessel may be determined relative to the reference leg in a conventionally known manner.
In order to passively maintain the constant height column of water in the reference leg, the reference leg includes a conventional cold condensing chamber at its top end which is joined to a downwardly inclined steam leg or pipe joined to the vessel. In this way, the steam in the pressure vessel flows upwardly through the steam leg and into the condensing chamber where it condenses to continually maintain the condensing chamber and reference leg joined thereto filled with water to a predetermined level, with any excess condensate in the condensing chamber spilling downwardly back into the vessel through the downwardly inclined steam leg.
A typical BWR has many ranges of water level monitoring, and therefore corresponding variable legs and differential pressure monitors. In one design for example, one variable leg includes a pressure tap in the pressure vessel at an elevation below the inlet tap for the steam leg near the bottom of the steam separator, to which is joined a conventionally known narrow range (NR) monitor calibrated for accurately measuring water level as it varies within a relatively small percentage of the total height of the vessel or from the normal vessel water level. Another variable leg includes a pressure tap located in the vessel below the NR pressure tap near the top of the active fuel in the reactor core, to which is joined a conventionally known wide range (WR) monitor effective for measuring water level from the normal level or top of the NR range down to about the WR pressure tap. Yet another variable leg is provided with a pressure tap in the vessel at an elevation below the WR pressure tap and below the bottom of the active fuel in the reactor core near the bottom of the pressure vessel to which is joined a conventionally known fuel zone range (FZR) monitor effective for monitoring water level in the vessel down to about the FZR pressure tap. The monitors conventionally join each of these variable legs to the common reference leg in order to monitor water level relative thereto based on the differential pressures monitored from which the water level may be conventionally determined relative to the known water level in the reference leg.
In this exemplary embodiment, the NR monitor is primarily used for feedwater control, certain reactor trips based on water level, and for the automatic depressurization system permissive. The WR monitor is used for reactor core trips associated with the main steamline isolation and emergency core cooling system. The FZR monitor is used for indicating water level in the core range, and for containment spray permissive in some plants. The three monitors are each calibrated for accurately monitoring water level within preferred ranges, with the ranges typically overlapping to ensure continuous water level monitoring throughout the entire elevation range as desired. Of course, various BWRs use various numbers of water level monitors for controlling operation of the reactor and various subsystems as required. However, all such conventional water level monitors use a variable leg joined to a common, constant reference leg by the differential pressure monitor for determining water level.
Observations at certain operating plants have shown that the vessel water level monitors may spuriously read falsely high water levels for short periods during slow depressurization of the pressure vessel in preparation for a maintenance outage. This temporary false increase in level measurement is also referred to as a notch in the indicated vessel water level which has been found to be plant dependent, with some plants not experiencing a notch, and other plants experiencing a notch having a duration from a few seconds to about a couple minutes, with the notch being observed at reactor vessel pressures less than about 450 psig (32 kg/cm.sup.2 g). The magnitude of the notch varies from about 4 to 8 inches (10 to 20 cm) for example. And, the frequency of occurrence of the notch also varies. The notch is undesirable since the accuracy of the water level is reduced and falsely reads high.